Of all the resources that plant species need to grow and develop, water is the most abundant and may also be the most limiting due to the large volume that a plant must absorb throughout its life cycle.
Original document published in:
Quarterly Letter
v. 31, Nos. 3 and 4 of 2009. July December.
http://www.cenicana.org/publicaciones/carta_trimestral/ct2009/ct3y4_09/ct3y4_09_p16-18.php
Fernando Muñoz Arboleda *
* Agronomist Engineer, Ph.D., Edafógo de Cenicaña {source}
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Introduction
Of all the resources that plant species need to grow and develop, water is the most abundant and may also be the most limiting due to the large volume that a plant must absorb throughout its life cycle. However, plants only retain approximately 3% of the total volume of water they absorb, the amount they use in photosynthesis and other metabolic processes. The 97% of the remaining water, whose main function is the transport of dissolved nutrients through the plant, ascends from the root to the surface of the leaves, where it is evaporated in the form of perspiration.
According to the metabolic pathway they use in the photosynthetic process of fixation of carbon dioxide (CO2), the plants are classified into C3 and C4. It has been estimated that for each CO molecule2 fixed, C3 plants must absorb, transport and evaporate approximately 500 water molecules into the atmosphere. C4 plants, such as sugarcane, are more efficient in water use and it is estimated that they require about 250 water molecules to fix a CO molecule2 (Taiz and Zeiger, 2006). From the above it can be deduced that imbalances in the flow of water through the plant, even if they are small, can cause serious problems in many cellular processes involved in the accumulation of dry matter, a parameter used to estimate the potential productivity of The crops
water in the floor
Water deficit in the soil is the main factor that prevents crops from reaching their productivity potential. Water affects the chemical form in which nutrients are found in the soil and when a moisture deficit occurs, the availability of those is reduced even though they are in sufficient quantities. In order for them to be absorbed by the root and transported through the plant to the places where they will be metabolized, the nutrients must be dissolved in the water present in the pores that form between the soil particles, that is, in the soil. Soil solution, in which water acts as a solvent and nutrients act as solute.
The amount of rainfall and its distribution over time in a given site or region may allow optimum yields to be achieved in the crops or may prevent it. Thus, the factors that limit productivity most occur when the volume of water due to rain or irrigation is less than the crop requirement, or when there is little water availability at times of maximum demand in combination with low capacity soils. to retain moisture in available form. In the same way, in soils with low capacity to evacuate excess water, productivity can be affected, even as negatively as it occurs when available water is deficient.
Although the total water content in a soil is very high, the water available for cultivation is always lower because the usable water is that between the points of field capacity and permanent wilting. The water content of the soil is defined as field capacity when it has been drained by gravitational force; To determine the field capacity, the amount of water retained is measured when an 1 / 3 suction of atmospheric pressure is applied to the soil. The physical definition of the permanent wilting point is the moisture content of a soil subjected to a suction of 15 atmospheres. Biologically, the wilting point is defined as the moisture content from which a plant no longer recovers from water deficiency damage, even though irrigation is applied.
The ability of a soil to retain water available to plants depends primarily on the texture of the soil. In general, clay soils can retain a greater amount of moisture than light or sandy soils. Water moves through the soil primarily by the mass flow generated by pressure gradients in the soil mass. In this way the water moves from regions of high moisture content, where there are macropores filled with water, to regions of lower moisture content, where there is only water in pores of smaller size. As the plant absorbs water from the soil, near the surface of the root an area is created where the osmotic potential is low, which generates a massive flow of water from nearby soil regions and with greater osmotic potential. .
Water movement from the ground to the leaves
To achieve effective water absorption there must be a good contact between the root surface and the soil. The contact area is maximized as the absorbent hairs grow, which penetrate between the soil particles. Absorbent hairs are extensions of the epidermal cells of the root that contribute to increase the root surface and the ability to absorb ions and water from the soil. It has been found that the surface of the absorbent hairs can represent up to 60% of the total root surface area (Taiz and Zeiger, 2006). Water enters the plant mainly through the area closest to the apex of the roots, where the layer of cells called exodermis that contains hydrophobic materials such as suberine has not yet developed, which accumulates in the older regions of the root and makes it relatively waterproof (Kramer and Boyer, 1995).
In order to be absorbed, the nutrients must be dissolved in the water in the pores that form between the soil particles. The presence and solubility of the mineral elements necessary for the development of the plants are determining factors so that the plant reaches the sufficiency of one or more nutrients or so that it suffers the deficiency of them. Likewise, when a mineral element is present in large quantities and is available in the soil solution, toxicity problems can occur.
The leaves are the main center of photosynthetic activity of the plant. To reach the leaves, water with dissolved nutrients must pass through different media (cell wall, cytoplasm, membranes and porous spaces), so that it uses transport mechanisms according to the medium; Among the main mechanisms are molecular diffusion (fast transport over short distances but extremely slow over long distances) and mass flow (very fast over long distances).
During the day the plant acts as an efficient suction pump that takes water with dissolved salts (soil solution) through the root and expels water vapor through the leaves. The water outlet in the form of steam creates a negative pressure gradient in the plant that, in combination with the capillarity effect in the xylem, allows water to be carried with dissolved nutrients from the root to the leaves. Thus, the force that causes the solution of the soil to enter the plant comes from the perspiration of water vapor through the open stomata found on the surface of the leaves (see illustration).
Stomata have two cells called guard cells that act as a valve that allows gas exchange between the leaf and the atmosphere. In recent studies it has been found that stomata are such specialized structures that they can be activated by different mechanisms; These are the cases of the concentration of potassium that opens the stomata in the morning and the concentration of sucrose, responsible for keeping them open the rest of the day while the plant performs its photosynthesis processes (Talbot and Zeiger, 1998). When the stomata are open, the air entering the leaf evaporates the film of water that covers the mesophilic cells and then the negative hydrostatic pressure that generates the suction necessary for the water in the xylem to rise to leaves; therefore, during the night, when the stomata close, perspiration and water entering the plant stop.
Under conditions of high relative humidity of the air, when the plant is not transpiring, a phenomenon known as root pressure can occur that occurs due to the accumulation of ions that generate positive hydrostatic pressure in the xylem. This pressure causes the phenomenon of gutting that is observed early in the morning in the form of drops on the edges of the leaves in some species of plants. Those drops commonly called dew drops are, in fact, sap drops, which is exuded through specialized pores called hydrates.
Effect of water on soil nutrients
The soil moisture deficit decreases the availability of nutrients even though they are in sufficient quantities. Plants require that nutrients be dissolved in the soil solution so that they can be absorbed and translocated to the places where they will be metabolized. Excess water or deficit in the root zone affects the chemical form in which nutrients are present in the soil.
Nitrogen is the nutrient that most limits the production of crops of non-leguminous species. Sugarcane, being a grass, depends on the nitrogen from the mineralization of the organic matter in the soil and the fertilizers applied as a complement to the nitrogen available in the organic matter. Nitrogen is absorbed by plants in the chemical form of ammonium (cation) or in the form of nitrate (anion) and the biological effect of absorbing it in one way or another can affect the physiological performance and productivity of crops. It has been found that in general plants require a combination of both forms of nitrogen, although the tendency is that a higher proportion of nitrate than ammonium is required.
The predominant form of nitrogen in the soil depends mainly on the moisture content of the soil. Thus, in soils with limited aeration due to excess moisture, nitrification is restricted (passage of ammonium to nitrate), because this process is carried out by exclusive aerobic bacteria. According to the above, it can be affirmed that sugarcane requires well-aerated soils to have a balanced nitrogen nutrition in terms of the nitrate / ammonium ratio, which allows the potential crop yield to be achieved. Also, that soils with excess moisture strongly decrease the productivity of sugarcane crops.
Bibliographic references
Kramer, PJ and Boyer, JS 1995. Water relations of plants and soils. Academic Press, San Diego, CA. 325 p.
Taiz, L. and Zeiger, E. (Eds.) 2006. Plant physiology Sinauer Associates, Inc. Publishers. Sunderland, MS. 764 p.
Talbot, LD and Zeiger, E. 1998. The role of sucrose in guard cell osmoregulation. Journal of Experimental Botany, 49: 329-337.
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